1. Field of the Invention
This invention relates to a device and method to assess and predict the stability of stored fuels.
2. Description of the Prior Art
As available petroleum crudes continue to decrease in quality and the amount of catalytically cracked stock used in middle-distillate fuels increases, there is a need for an accelerated stability test which is capable of reliably assessing and predicting the long term storage stability of middle distillate fuels such as Grade No. 1D and Grade No. 2D diesel fuels.
Currently used methods for fuel storage stability assessment assume Arrhenius-like behavior for typical fuels being oxidized in the temperature region from 20.degree. C. to 95.degree. C. under laboratory accelerated tests (Hardy, Dennis R., Hazlett, Robert N., Giannini, R., and Strucko, R., "Stability Measurements of Commercial Marine Fuels from a Worldwide Survey" SAE Technical Paper Series, No. 860895, 1986) . For each 10.degree. C. rise in temperature, there is an approximate doubling in reaction rate. However, these methods suffer from a variety of drawbacks (Hardy, D. R., Beal, E. J., Hazlett R. N., and Burnett, J. C., "Assessing Distillate Fuel Storage Stability By Oxygen Overpressure," Proceedings of the Third International Conference on Stability and Handling of Liquid Fuels, 1988). Some tests take too long. For example, lower temperature bottle tests are generally good indicators of storage stability of a particular fuel. However, meaningful results require storage at 43.degree. C. for between 12 and 18 weeks. On the other hand, bottle storage tests at temperatures of 80.degree. C. and above can be completed in a reasonably short time. But, these tests are generally poor indicators of actual ambient fuel reactions that lead to insoluble products.
Another test widely used as a rapid assessment of fuel storage oxidative stability is the ASTM D2274 method. This method accelerates the oxidation of fuels (and thus decreases the time required for fuel stability measurement) by bubbling oxygen through the fuel. It is not a good predictor of storage stability for freshly refined middle-distillate fuels that contain any catalytically cracked stocks. The recommended fuel incubation time of 16 hours is too short for many fuels and leads to misleading results and very small amounts of total insolubles which are hard to quantify. Because of these results, this test allows a potentially unstable fuel to pass the test.